1. Field of the Invention
This invention relates to an optical fiber and a method for producing the same, and more particularly to an optical fiber of low transmission loss and a method for producing the same.
2. Brief Description of the Prior Art
The optical fiber which is the transmission medium of optical communication exhibits a low transmission loss. In order to produce the optical fiber of low transmission loss, impurities such as heavy metal ions and hydroxyl ions, which induce light absorption, must be prevented from mixing into the core part of the optical fiber, i.e. the inner portion, and the cladding part of the optical fiber, i.e. a portion located between the core part and the glass tube providing the outer portion of the optical fiber.
To this end, it has, heretofore, been common practice that an optical fiber preform is produced by a chemical vapor reaction employing a starting raw material of high purity and a carrier gas of high purity so as to prevent the mixing of the impurities from the raw material, and that a pure SiO.sub.2 layer to serve as an impurity diffusion-preventing layer is provided between a glass tube and a cladding layer or that a thick cladding layer to function also as an impurity diffusion-preventing layer is provided between the glass tube and the core, whereby the diffusion of the impurities from within the glass tube or from outside it to the core part is prevented. In the former method of providing the pure SiO.sub.2 layer, a high temperature of about 1,700.degree. C. is required for the deposition of the SiO.sub.2 layer. Unfavorably, the glass tube is subjected to deformation due to this high temperature. In the latter method of providing the thick cladding layer, it is necessary to make the thickness of the cladding layer about 10-15 .mu.m. This leads to the difficulty that the structures of optical fibers, which can be fabricated, are restricted.
Known techniques related to this invention are described in Japanese Patent Application Laid-Open Specifications Nos. 48-73523, 50-120352, 50-51338 and 51-3650.